As is well-known in the art, a spectrum analyzer continuously sweeps the local signal frequency which is mixed in the measuring frequency band of an input signal to sequentially convert respective frequency components in the measuring frequency band into intermediate-frequency signals of certain frequencies (difference frequencies) and displays their levels on a display screen with the abscissa representing frequency. Moreover, such a spectrum analyzer is usually designed to permit the observation of the waveform of a desired frequency component of the input signal in the time domain, and in such a waveform observation, the local signal frequency is fixed at a value so that the frequency component desired to observe may be detected as an intermediate-frequency signal, and the amplitude envelope of the intermediate-frequency signal thus obtained is displayed on the display screen with the abscissa representing the time axis.
FIG. 1 is a block diagram showing the basic construction of such a conventional spectrum analyzer. In the case where this device is used in a frequency analysis mode (a frequency sweep mode), a select switch 17 selects an output ramp voltage VR of a ramp address generator 23 and provides it to a local oscillator 16, and a select switch 29 selects, for example, the output of a trigger signal generator 26 to supply the ramp address generator 23 with a trigger signal Tr generated by the trigger signal generator 26 on the basis of a demodulated signal of a signal to be measured Sx applied to an input terminal 11. A sweep control signal SC is provided to an input terminal 27. The signal Sx applied to the input terminal 11 is frequency mixed by a mixer 12 with a local signal SL from the local oscillator 16, and the difference frequency component is extracted by an intermediate-frequency filter (which is a band-pass filter and will hereinafter be referred to as an IF filter) 13.
In the frequency analysis mode, the oscillation frequency of the local oscillator 16 is swept over a certain range by the ramp voltage VR from the ramp voltage generator 23, and consequently, respective frequency components in the measuring frequency band of the input signal are sequentially obtained as intermediate-frequency signals at the output of the IF filter 13. The output of the IF filter 13 is logarithmically amplified by a logarithmic amplifier 14 and the amplitude of its output is envelope detected by a detector 15. The detected output level is converted by an A/D converter 18 into a digital value upon each application of a high-speed clock CK and is stored in a memory 19 by an address AD which is generated by the ramp address generator 23 upon each application of the clock CK. The signal data thus stored in the memory 19 is transferred therefrom to a memory for image display use 20 by a read/write address for transfer use TAD supplied from a controller 31. Upon completion of the transfer, an image signal generator 21 repeatedly reads out the signal data from a series of addresses in a data storage area of the memory 20 for each horizontal scanning line corresponding to each height (corresponding to the signal level) on the display screen. When the value (the level value) of signal data is present which agrees with the level corresponding to respective horizontal scanning line number, the image signal generator generates an image signal which goes high at that position (corresponding to the time position) on the horizontal scanning line corresponding to the address value of the signal data but remains low at other positions. The image signal is applied to a raster scanning display 22 for display thereon. The abscissa of the display screen represents frequency and the ordinate represents level.
To perform the above-described operation, the ramp address generator 23 is initialized by a reset signal RST from the controller 31, and while the sweep control signal SC applied to the input terminal 27 is at the H-logic level, it counts the high-speed clock pulses CK from predetermined minimum to maximum value which are provided as data DATA from the controller 31 and sequentially outputs the count values as addresses AD and, at the same time, it converts each count value into an analog value for output as the ramp voltage VR. When the maximum value of the address AD is reached, the ramp address generator yields and applies an interrupt signal INT to the controller 31. When supplied with the interrupt signal INT, the controller 31 supplies the read/write address for data transfer used TAD to the memory 19, from which the signal data is transferred to the memory 20. Upon completion of the data transfer, the controller generates a trigger enable signal TE, putting the ramp address generator 23 in the state in which it is ready for counting again the clock pulses CK from the minimum value.
In the case of observing the waveform of a desired frequency component of the input signal in the time domain (This mode of observation will hereinafter be referred to as a zero-span mode.), a variable voltage source 24 is selected by the select switch 17 and a desired fixed voltage is supplied to the local oscillator 16, from which the local signal SL of a fixed frequency is applied to the mixer 12. Hence, in this instance, the output of the detector 15 becomes an envelope waveform of the amplitude of the specified frequency component in the input signal which corresponds to the lapse of time. On the other hand, the select switch 29 is connected to, for example, the output of the trigger signal generator 26. For example, when the signal to be measured Sx is a burst wave, the output level of the detector 15 is compared by a comparator 25 with a predetermined level to detect the rise of each burst and the detected output is applied to the trigger signal generator 26 to generate therefrom the trigger pulse signal Tr of a fixed width. Thus the trigger signal Tr which is synchronized with each burst can be obtained.
Upon each application of the trigger signal Tr, the ramp address generator 23 counts the clock pulses CK from predetermined minimum to maximum value and provides each count value, as the address AD, to the memory 19. In consequence, the detected outputs of the detector 15 are sequentially converted into digital values as in the above and they are stored in those areas of the memory 19 specified by a sequence of addresses AD from the ramp address generator 23, after which the signal data thus stored is transferred to the memory 20. The signal data read out of the memory 20 is converted by the image signal generator 21 into image signals, which are displayed on the display screen of the display 22. The abscissa of the display screen represents time and the ordinate represents level.
In the above it is described that the fixed voltage which is applied to the local oscillator 16 in the zero-span mode is obtained from the variable voltage source 24 via the select switch 17, but in the actual spectrum analyzer such a variable voltage source 24 is not provided but instead provision is made merely for stopping the sweep of the ramp voltage VR at a desired voltage value and for applying the fixed voltage to the local oscillator 16. To facilitate a better understanding of operations in the frequency analysis mode and the zero-span mode, however, the above description has been given on the assumption that the select switch 17 and the variable voltage source 24 are used.
The spectrum analyzer has sweep control terminals 27 and 28, in addition to the input terminal 11 for the input of the signal Sx as described above. While the sweep control signal SC (shown in FIG. 2, Row B, for instance) which is applied to the sweep control terminal 27 is at the H-logic level, the ramp address generator 23 linearly increases its output voltage VR toward a predetermined maximum value at a fixed gradient as shown in FIG. 2, Row C, and hence in this while the oscillation frequency of the local oscillator 16 linearly increases. When the sweep control signal SC goes down to the L-logic level, the ramp address generator 23 stops the sweep of its output voltage VR, and consequently, the frequency sweep of the local oscillator 16 is stopped. Thus, the use of the sweep control terminal 27 permits control of the local oscillator 16 from the outside to continue or stop its frequency sweep operation. On the other hand, upon each application of an external trigger pulse EXTr to the sweep control terminal 28, the ramp address generator 23 once sweep the output voltage VR from the minimum to the maximum value.
The sweep control terminal 27 is used in the case of analyzing frequency components contained in a carrier CY of a burst wave such as depicted in FIG. 2, Row A. That is, in the case where the burst wave, which is the signal to be measured Sx, is input into the spectrum analyzer and its frequency components are analyzed over one continuous range of time containing a plurality of bursts, frequency spectra SP.sub.PU of pulses of a burst modulation wave as well as a frequency spectrum SP.sub.CY of the carrier CY are displayed as shown in FIG. 3. This gives rise to a disadvantage that the presence or absence of harmonics of the carrier CY cannot be observed.
To avoid this, it is customary in the prior art to generate, outside the spectrum analyzer, the sweep control signal SC synchronized with the burst wave as shown in FIG. 2, Row B and input the sweep control signal SC to the sweep control terminal 27 of the spectrum analyzer, effecting control to sweep the oscillation frequency of the local oscillator 16 during the existence of the carrier CY of the burst wave and stop the frequency sweep when the carrier CY does not exist. By this control, only the frequency spectrum SP.sub.CY of the carrier CY is displayed on the display 22 of the spectrum analyzer as depicted in FIG. 4. Incidentally, this control state is referred to as gated sweep.
Thus the prior art has the disadvantage of involving the use of a circuit for generating the sweep control signal SC synchronized with the burst wave, because the sweep control signal must be produced outside the spectrum analyzer. Further, since signals to be measured range from low-frequency to ultrahigh-frequency signals of the gigahertz band, it is difficult to obtain the sweep control signal SC by direct waveform shaping of the burst wave.
In the mode in which to display the time-domain waveform of the input signal (i.e. in the zero-span mode), since the ramp address generator 23 generates a sequence of addresses AD (and consequently the ramp voltage VR shown in FIG. 5, Row C) upon each application of the trigger signal Tr (or EXTr), the loading of data into the memory 19 starts in synchronization with the external trigger signal EXTr which is applied to the sweep control terminal 27 or the trigger signal Tr which is provided from the trigger signal generator 26 as referred to above. Hence, if the signal to be measured Sx is a burst wave as shown in FIG. 5, Row A, the trigger signal Tr is not always generated in the duration of the burst as depicted in FIG. 5, Row B and, according to the temporal relation between the trigger signal Tr and the burst wave CY, a no-signal period Ts may sometimes exist from the timing of the trigger signal Tr to the period in which the carrier CY of the burst wave is present. When the generation of the ramp voltage VR (and consequently the address AD) is initiated in response to each trigger signal Tr as shown in FIG. 5, Row C, the A-D converted output obtained in the no-signal period Ts is also loaded in the memory 19 and then transferred therefrom to the memory 10, resulting in a disadvantage that the carrier CY desired to observe primarily cannot be displayed all over the display screen.
A first object of the present invention is to provide a spectrum analyzer which is equipped with a function of generating the sweep control signal and adapted to execute the gate sweep mode when supplied, from the outside, with a burst-like signal to be measured and a synchronizing signal synchronized therewith.
A second object of the present invention is to provide a spectrum analyzer which is capable of displaying, in the zero-span mode (i.e. in the time-domain waveform display mode), the waveform of the signal to be measured on a display, starting at a desired timing of the signal.